: Vesicular stomatitis virus (VSV) has been developed as a live mucosal vaccine vector and is highly effective in an influenza model system. The goal of this competitive renewal is to continue development of VSV/HIV hybrid viruses as HIV vaccines using a mouse model for initial screening of promising approaches. New strategies designed to enhance both neutralizing antibody and cell-mediated immune responses to HIV-1 proteins are being developed. Initial studies have established that complete HIV envelope protein (HIV Env) can be expressed in VSV/HIV hybrids and that incorporation of HIV Env into the VSV envelope enhances the antibody response to Env. Both antibody and strong cytotoxic T lymphocyte (CTL) responses to HIV Env have been seen in mice and in rhesus macaques infected with the vectors. The first aim is to test new strategies for enhancing neutralizing antibody responses to HIV Env. One strategy is based on results of others describing SIV Env mutants lacking glycosylation sites which generated high neutralizing antibody titers to the parental virus. A set of HIV env mutants lacking increasing numbers of N-linked glycans has been generated in Dr. Rose's laboratory. These will be expressed in VSV recombinants and tested in mice for induction of HIV neutralizing antibody. Additional studies will determine the effectiveness of VSV vectors encoding both Env and Gag proteins of HIV, and producing large amounts of HIV virus-like particles containing Env and Gag proteins. Boosting of the immune responses using VSV vectors is limited because antibodies to the single vector glycoprotein (G) are developed after the first immunization. A new vector system using a different G protein in the vector has been devised and will be tested in mice. The second aim is to quantitate the cellular immune responses to HIV Env and Gag proteins expressed in VSV vectors, and to investigate approaches to enhancing these responses. The third aim is to continue development of live VSV/HIV surrogate viruses that have a total replacement of the VSV envelope protein by HIV Env-G hybrid proteins. These viruses might be directly applicable as vaccines or could be used in boosting immune responses to HIV.